The present invention relates to the field of semiconductor devices. More particularly, the present invention relates to a crystal-less oscillator circuit with trimmable analog current control for providing increased frequency stability.
Oscillator circuits are used with digital electronic circuits (e.g., microcontrollers and microprocessors) for a variety of uses including generation of a clock signal for synchronizing, pacing and coordinating the operations of the digital electronic circuit. Some common forms of oscillator circuits are crystal oscillator circuits and voltage controlled oscillators (VCO) circuits.
A crystal oscillator circuit is an oscillator circuit that uses a quartz crystal to generate a frequency. Typical crystal oscillators generate a frequency of substantially 32 KHz. Often, as many applications require a frequency different than 32 KHz (for example 24 MHz or 48 MHz), a phase locked loop circuit is used to lock to a crystal oscillator to a VCO circuit to generate a frequency of a higher accuracy. Crystal oscillator circuits generally output a fixed frequency and are very precise.
A VCO circuit is an oscillator that can be tuned over a wide range of frequencies by applying a voltage (tuning voltage) to it and does not require a crystal. VCOs are typically less costly than crystal oscillators, but not as stable. For example, the frequency generated by a VCO may vary xc2x120%. Process variation, temperature variation and voltage variation are primarily responsible for this instability.
The required frequency stability of an oscillator depends on the specific application. For example, a computer system in a standard operating mode typically requires a precise frequency while a computer system in a sleep mode typically requires a less precise frequency.
It is desirable to use a crystal-less oscillator where possible for a number of reasons. An oscillator without a crystal requires fewer parts, thus saving room in a system using a microcontroller. Furthermore, requiring fewer parts reduces the cost of the oscillator, thus reducing the cost of the overall system. As many applications do not require the precise timing accorded by a crystal oscillator, it may be beneficial in certain situations to use a crystal-less oscillator.
Currently, there is a large gap between the precision offered by a crystal oscillator and that offered by a voltage controlled oscillator (e.g., a crystal-less oscillator). Applications that require a moderate degree of stability require a crystal oscillator circuit, as current VCO circuits do not offer the stability required.
Furthermore, certain applications only require the stability offered by a crystal oscillator part of the time. It is desirable to have an oscillator that offers an improved degree of precision over current crystal-less oscillators while offering the option of phase locking to an external crystal oscillator for use when a high degree of precision is necessary.
Accordingly, a need exists for an oscillator circuit that does not require a crystal but still offers an improved degree of frequency stability above current voltage controlled oscillators. A need also exists for an oscillator circuit that satisfies the above need, and allows for stable operation over process variations and temperature variations by having a trimmable current control. A need also exists for an oscillator circuit that satisfies the above needs, and operates in a phase locked loop that can lock to an external crystal for use in applications requiring high precision.
A crystal-less oscillator circuit with trimmable current control is presented. In one embodiment, the present invention provides an oscillator circuit comprising a digital to analog converter circuit for generating a current, a band gap reference circuit for generating a voltage, and a relaxation oscillator circuit for creating a frequency based on the current and the voltage.
In one embodiment, the digital to analog converter circuit comprises a trimmable current control for generating a current. The trimmable current control may comprise a plurality of trimmable components. In one embodiment, the trimmable current control comprises four trimmable components.
In another embodiment, the present invention provides a phase locked loop circuit comprising a phase detector circuit and the aforementioned oscillator circuit. In another embodiment, the present invention provides a microcontroller comprising a phase locked loop circuit comprising a phase detector circuit and the aforementioned oscillator circuit.
The present invention provides an oscillator with stable frequency without the use of a crystal oscillator. The trimmable current control allows for stable operation of the oscillator over process variations, temperature variations and voltage variations. In one embodiment, the oscillator circuit of the present invention operates in a phase locked loop that can lock to an external crystal for use in applications requiring high precision.
These and other objects and advantages of the present invention will become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.